Ultrasound acoustic assemblies and methods of manufacture

ABSTRACT

An ultrasound acoustic assembly includes a number of ultrasound acoustic arrays, each array comprising an acoustic stack comprising a piezoelectric layer assembled with at least one acoustic impedance dematching layer and with a support layer. The acoustic stack defines a number of dicing kerfs and a number of acoustic elements, such that the dicing kerfs are formed between neighboring ones of the acoustic elements. The dicing kerfs extend through the piezoelectric layer and through the acoustic impedance dematching layer(s) but extend only partially through the support layer. The ultrasound acoustic assembly further includes a number of application specific integrated circuit (ASIC) die. Each ultrasound acoustic array is coupled to a respective ASIC die to form a respective acoustic-electric transducer module. Methods of manufacture are also provided.

BACKGROUND

The invention relates generally to ultrasonic transducer assemblies and,more particularly, to methods of fabricating ultrasonic transducerassemblies.

Ultrasonic transducer assemblies are typically employed in applicationsincluding non-destructive evaluation (NDE) and medical diagnosticimaging, such as ultrasound applications and computed tomography (CT).The ultrasonic transducer assembly generally includes an array ofultrasonic transducers coupled to an electronics array. As explained incommonly assigned U.S. Pat. No. 7,892,176, Robert Wodnicki et al.“Monitoring or imaging system with interconnect structure for large areasensor array,” which is incorporated by reference herein in itsentirety, transducer arrays in ultrasound probe assemblies typicallyspan an area no larger than about 20 cm². For new medical applications,such as screening for internal bleeding and tumors, much larger arrays,on the order of 300 cm², are required. In non-medical applications evenlarger arrays may be desired.

The ultrasonic transducer array generally includes hundreds or thousandsof individual transducers. Piezoelectric transducers (for example, PZT)are a widely used type of ultrasonic transducer. Piezoelectric sensorsgenerally include a piezoelectric material capable of changing physicaldimensions when subjected to electrical or mechanical stress. Inaddition, piezoelectric sensors may include layers of matching materialsand damping materials.

Similarly, the electronics array includes hundreds or thousands ofintegrated interface circuits (or “cells”) which are electricallycoupled to provide electrical control of the transducers for beamforming, signal amplification, control functions, signal processing,etc. In particular, each transducer sub-array or chip in the transducerarray is typically coupled to an integrated circuit chip to provideindividual control of each sensor.

Such large arrays may be formed by tiling of a large number oftransducer modules in rows and columns. The array may be one dimensional(1D) (a linear array or row of acoustic elements) for two-dimensional(2D) imaging. Similarly, the array may be a 2D array for volumetricimaging. Each transducer module comprises a subarray of transducer cellsand an integrated circuit coupled to the subarray. Fabricating thetransducer array and the electronics array, and coupling the two arraystogether, provides a number of design challenges. For example,performance of a large area transducer array is significantly degradedwhen there are significant variations in the spacing between modules.

Further, for many current medium-sized arrays, significant mutingchallenges exist to bring the connections from the nearby electronicsinto the sensor array. In addition, these arrays are limited by theavailable routing density and also by the parasitic capacitances of thetraces.

It would therefore be desirable to provide acoustic arrays withcontrolled spacing between the transducer elements. It would further bedesirable to provide acoustic arrays that can be readily assembled intoa larger assembly and coupled with front end electronics in anarrangement with low parasitic capacitance. It would also be desirableto provide methods of manufacturing the acoustic arrays and the overallassemblies.

BRIEF DESCRIPTION

One aspect of the present invention resides in an ultrasound acousticassembly that includes a number of ultrasound acoustic arrays, whereeach array includes an acoustic stack comprising a piezoelectric layerassembled with at least one acoustic impedance dematching layer and witha support layer. The acoustic stack defines a number of dicing kerfs anda number of acoustic elements, such that the dicing kerfs are formedbetween neighboring ones of the acoustic elements. The dicing kerfsextend through the piezoelectric layer and through the acousticimpedance dematching layer(s) but extend only partially through thesupport layer. The ultrasound acoustic assembly further includes anumber of application specific integrated circuit (ASIC) die. Eachultrasound acoustic array is coupled to a respective one of the ASIC dieto form a respective acoustic-electric transducer module.

Yet another aspect of the present invention resides in a method ofmanufacturing an ultrasound acoustic assembly. The manufacturing methodincludes assembling a piezoelectric layer with at least one acousticimpedance dematching layer and with a support layer to form an acousticstack. The manufacturing method further includes dicing the acousticstack to form a number of acoustic elements, such that a number ofdicing kerfs are formed between neighboring ones of the acousticelements. The dicing kerfs extend only partially through the supportlayer. The manufacturing method further includes connecting each of thediced, acoustic stacks to a respective one of a number of applicationspecific integrated circuit (ASIC) die, to form a respectiveacoustic-electric transducer module.

Another aspect of the present invention resides in a method ofmanufacturing an ultrasound acoustic assembly. The manufacturing methodincludes depositing an under-bump metallization (UBM) on a number ofacoustic impedance dematching regions. The manufacturing method furtherincludes disposing a number of conductive bumps on a substrate or on anumber of application specific integrated circuit (ASIC) die. Themanufacturing method further includes disposing the acoustic impedancedematching regions on the substrate or on the ASIC die, where theconductive bumps are disposed between the acoustic impedance dematchingregions and the substrate or ASIC die. The manufacturing method furtherincludes performing a reflow operation, such that a number of uniformelectrical connections are formed between the acoustic impedancedematching regions and the respective ASIC die or substrate, anddisposing at least one piezoelectric region on the acoustic impedancedematching regions to form respective acoustic stacks.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic cross-sectional view of an example acoustic stack;

FIG. 2 shows the acoustic stack of FIG. 1 after dicing;

FIG. 3 shows the diced, acoustic stack of FIG. 2 after bumping and afterthe common electrode has been deposited;

FIG. 4 is a schematic cross-sectional view of an example acoustic stack,where the acoustic impedance matching layer has been bump plated priorto dicing;

FIG. 5 is a schematic cross-sectional view of an exampleacoustic-electric transducer module;

FIG. 6 is a schematic cross-sectional view of an exampleacoustic-electric transducer module, where the support layer waspartially removed;

FIG. 7 is a schematic cross-sectional view of an example ultrasoundacoustic assembly;

FIG. 8 is a schematic cross-sectional view of an example laminatedacoustic assembly;

FIG. 9 is a schematic cross-sectional view of the example laminatedacoustic assembly of FIG. 8 disposed on a substrate;

FIG. 10 is a schematic cross-sectional view of the assembly of FIG. 9after singulation;

FIG. 11 is a schematic cross-sectional view of the assembly of FIG. 9coupled to an ASIC die;

FIG. 12 is a schematic cross-sectional view of the example laminatedacoustic assembly of FIG. 8 disposed directly on an ASIC die; and

FIG. 13 is a schematic cross-sectional view showing a gold layerdisposed on a polished upper surface of an acoustic impedance dematchinglayer to facilitate the subsequent laminations shown in FIGS. 8-12.

DETAILED DESCRIPTION

The terms “first,” “second,” and the like, herein do not denote anyorder, quantity, or importance, but rather are used to distinguish oneelement from another. The terms “a” and “an” herein do not denote alimitation of quantity, but rather denote the presence of at least oneof the referenced items. The modifier “about” used in connection with aquantity is inclusive of the stated value, and has the meaning dictatedby context, (e.g., includes the degree of error associated withmeasurement of the particular quantity). In addition, the term“combination” is inclusive of blends, mixtures, alloys, reactionproducts, and the like.

Moreover, in this specification, the suffix “(s)” is usually intended toinclude both the singular and the plural of the term that it modifies,thereby including one or more of that term (e.g., “the array” mayinclude one or more arrays, unless otherwise specified). Referencethroughout the specification to “one embodiment,” “another embodiment,”“an embodiment,” and so forth, means that a particular element (e.g.,feature, structure, and/or characteristic) described in connection withthe embodiment is included in at least one embodiment described herein,and may or may not be present in other embodiments. Similarly, referenceto “a particular configuration” means that a particular element (e.g.,feature, structure, and/or characteristic) described in connection withthe configuration is included in at least one configuration describedherein, and may or may not be present in other configurations. Inaddition, it is to be understood that the described inventive featuresmay be combined in any suitable manner in the various embodiments andconfigurations.

A method of fabricating one or more ultrasound acoustic arrays 40 isdescribed with reference to FIGS. 1-6. As indicated, for example, inFIG. 1, the fabrication method includes assembling a piezoelectric layer42 (for example, PZT) with at least one acoustic impedance dematchinglayer 41 and with a support layer 2 to form an acoustic stack 18. Forparticular arrangements, the piezoelectric layer 42 comprises singlecrystal PZT. Non-limiting examples of the dematching layer includesdense, high modulus metals, such as molybdenum or tungsten, and highdensity ceramics, such as tungsten carbide. For particularconfigurations, the acoustic impedance dematching layer 41 comprises atungsten carbide high impedance dematching layer 41. The dematchinglayer 41 has an acoustic impedance greater than that of thepiezoelectric layer 42. For the configuration shown in FIG. 1, thesupport layer 2 may have an acoustic impedance between that of water andthe PZT layer 42.

For the process illustrated by FIGS. 1-3 and 5, the fabrication methodfurther includes dicing the acoustic stack 18 to form a number ofacoustic elements 4, such that a number of dicing kerfs 14 are formedbetween neighboring ones of the acoustic elements 4. As indicated, forexample, in FIGS. 2, 3 and 5, the dicing kerfs 14 are deep enough toseparate the acoustic elements 4 from one another. However, for theillustrated arrangement, the dicing kerfs 14 extend only partiallythrough the support layer 2, in order to maintain a common mechanicalsubstrate support for the pillars in the array by means of remainingsupport layer (for example, graphite) material. The dicing may beperformed using, for example, a standard dicing saw or using laserdicing. Alternatively, a deep reactive ion or other semiconductor typeetch could be used with an intervening masking step to separate theacoustic elements. As used here. “dicing” should be understood toencompass mechanical dicing techniques (saw, laser etc.), as well aschemical separation techniques (various etching processes).

For particular processes, the support layer 2 comprises a graphitesupport layer 2. Other candidate materials for support layer 2 include,without limitation, ceramics, silicon, flexible organic polymers, metalfilled graphite, ceramic powder filled epoxy, glass, and glass-ceramics.

For the illustrated techniques, the fabrication method further includesdepositing an under-bump metallization (UBM) 33 on the acousticimpedance dematching layer 41. See, for example, FIG. 1. Typically theUBM 33 is deposited on the acoustic impedance dematching layer 41 priorto assembling the acoustic stack. Example UBM may comprisetitanium-nickel-gold. However, other metallization could be used aswell, including thick gold, or other metals. Example techniques forapplying the UBM include, without limitation, plating, sputtering, andevaporative processes. In addition, the UBM pads could be continuousacross the entire top surface of each acoustic element 4 or they couldalso be patterned into smaller square or circular pads, for exampleusing a lift-off process or by shadow-masking. More particularly, theUBM 33 may be patterned lithograhically before attaching solder bumps35. Beneficially, the required solder bump can be smaller for apatterned pad, while still maintaining the required standoff height forassembly.

For particular processes, the fabrication method further includes bumpplating the acoustic impedance dematching layer 41 to form a number ofraised electrical contacts 34, as shown, for example, in FIG. 3. Forexample, solder bumps 35 may be formed on the UBM 33 to form theelectrical contact 34. Bumping may be accomplished using standardtechniques including electroplating, screen printing, transferprocesses, or solder-jetting. The material for bumping may be anysuitable material such as eutectic tin/lead solder or a lead-freesolder. In certain applications, indium bumps, low temperature bismuthalloy bumps, or other techniques may also be used. For particularconfigurations, it is advantageous to use a low temperature solder, suchas bismuth or indium, in order to avoid the need to subsequently re-polethe piezoelectric layer 42.

In one non-limiting example, bump height is not critical, and a platedheight of about 25-50 micrometers can be used. For the processesillustrated in FIGS. 2-4, the bump plating is performed after theacoustic stack 18 has been assembled. That is, the bumps 35 are formedafter the acoustic stack 18 has been assembled. As noted above, the UBM33 is typically applied prior to assembling the acoustic stack. However,for certain processes (not shown), the acoustic impedance dematchinglayer 41 could be bump plated prior to assembling the acoustic stack 18.Typically though, the acoustic stack 18 will be assembled prior to thebump plating. For the process shown in FIG. 4, the bump plating isperformed before dicing the acoustic stack 18. Beneficially, bumping thestacks prior to dicing facilitates handling. However, the bump platingcould be performed after the dicing is completed, as shown, for examplein FIGS. 2 and 3. For example the UBM could be applied prior toassembling the acoustic stack 18, and the bumps 35 could be formed afterthe acoustic stack 18 has been diced.

For the arrangement shown in FIG. 3, the fabrication method furtherincludes assembling a common ground electrode 6 to the diced, acousticstack 18. For example, a suitable metal electrode 6 may be attached tothe support layer 2, along with an outer matching layer 8 attached toelectrode 6, to provide a common ground electrode 6 for all of theacoustic elements 4. Non-limiting examples of suitable materials for theouter matching layer 8 include ABS plastic, polyethylene, polystyrene,and unfilled epoxy. Other materials with similar acoustic impedances maybe used as well.

For particular processes, the fabrication method further includes atleast partially removing the support layer 2 to further acousticallyseparate the acoustic elements 4 from one another, as indicated, forexample, in FIG. 6. The optional removal of at least part of the supportlayer 2 would be performed prior to the assembly of the electrode 6 tothe acoustic stack 18.

In addition to the process steps described above, additional processingmay be performed. For example, prior to lamination to the piezoelectriclayer 42, a tungsten carbide acoustic impedance dematching layer 41 mayundergo sputtering of a layer of titanium (not shown) in order toprovide a seed layer (not shown) for the UBM 33. Next, a layer of nickelmay be evaporated on the tungsten carbide acoustic impedance dematchinglayer 41 in order to provide the proper pad for attaching the solderballs 35. This is followed by a very thin flash layer of gold (notshown) to prevent oxidation of the nickel. The tungsten carbide acousticimpedance dematching layer 41 containing the UBM 33 may then be bondedto the piezoelectric layer 42 and a graphite support layer 2 as thin,solid slabs of material bonded with an adhesive. Suitable nonconductiveadhesives include, without limitation epoxy, polyurethane, and acrylateadhesives and may be bonded using high pressure. Alternately conductiveand anisotropically conducting adhesives may be used to bond thedematching layer to the piezoelectric layer.

Beneficially, the above described fabrication method facilitates thefabrication ultrasound acoustic arrays 40 in a batch process, whichhelps reduce overall manufacturing costs for tiled modular acousticassemblies 100, which are described below.

An ultrasound acoustic array 40 is described with reference to FIGS.1-6. As indicated, for example, in FIGS. 1-3, the ultrasound acousticarray 40 includes an acoustic stack 18 (FIG. 1) comprising apiezoelectric layer 42 assembled with at least one acoustic impedancedematching layer 41 and with a support layer 2. The layers 2, 41, 42 aredescribed above. For particular arrangements, the piezoelectric layer 42comprises single crystal piezoelectric (for example, single crystalPMN-PT). As indicated, for example, in FIGS. 2 and 3, the acoustic stack18 defines a number of dicing kerfs 14 and a number of acoustic elements4, such that the dicing kerfs 14 are formed between neighboring ones ofthe acoustic elements 4. As indicated, for example, for the exampleconfigurations shown in FIGS. 2 and 3, the dicing kerfs 14 extendthrough the piezoelectric layer 42 and through the acoustic impedancedematching layer(s) 41 but extend only partially through the supportlayer 2. Beneficially, this dicing configuration provides a commonmechanical substrate support for the pillars in the array (i.e., theacoustic elements 4) by means of remaining support layer (for example,graphite) material.

As indicated, for example, in FIGS. 2 and 3, the ultrasound acousticarray 40 may further include an under-bump metallization (UBM) 33connected to the acoustic elements 4. As shown, for example, in FIGS. 2,3 and 5, the UBM 33 is segmented, with each segment of the UBM 33attached to an acoustic element 4. For the example configurations shownin FIGS. 3-6, the ultrasound acoustic array 40 further includes a numberof conductive bumps 35 connected to the UBM 33 to form a number ofraised electrical contacts 34 (FIG. 3) for the acoustic elements 4.Suitable materials for conductive bumps 35 are provided above.Alternative means for attaching the dematching layer may also beemployed.

For the configuration shown in FIGS. 3 and 5, the ultrasound acousticarray 40 further includes a common ground electrode 6 connected to thediced, acoustic stack 18. For example and as discussed above, a suitablemetal electrode 6 may be attached to the support layer 2, along with alayer of ABS plastic 8 or other suitable material for the outer matchinglayer 8 attached to the electrode 6, to provide a common groundelectrode 6 for all of the acoustic elements 2.

Beneficially, support layer 2 can function as a carrier providingstructural stability to the individual acoustic pillars (elements 4)prior to assembly of the ultrasound acoustic array 40 to the interposer50 (as described below). Support layer 2 allows the pillar arrays to behandled and for automated pick-and-place assembly, as discussed below.As noted above, the ultrasound acoustic arrays 40 can be fabricated in abatch process, which helps reduce overall manufacturing costs for largearea acoustic arrays. Further, the resulting ultrasound acoustic arrays40 may be tested individually, prior to assembly. This helps to ensure ahigh yield rate on the subsequently assembled large area acousticarrays.

An ultrasound acoustic assembly 100 embodiment of the invention isdescribed with reference to FIGS. 1-5 and 7. As shown for example inFIG. 7, the ultrasound acoustic assembly 100 includes a number ofultrasound acoustic arrays 40. As described above, each array comprisingan acoustic stack 18 (FIG. 1) comprising a piezoelectric layer 42assembled with at least one acoustic impedance dematching layer 41, forexample, a tungsten carbide high impedance dematching layer 41, and witha support layer 2, for example a graphite support layer 2. The layers 2,41, 42 are described above. As shown in FIGS. 2 and 3, the acousticstack 18 (FIG. 1) defines a number of dicing kerfs 14 and a number ofacoustic elements 4, such that the dicing kerfs 14 are formed betweenneighboring ones of the acoustic elements 4. As indicated in FIGS. 2 and3, for example, the dicing kerfs 14 extend through the piezoelectriclayer 42 and through the acoustic impedance dematching layer(s) 41 toseparate the acoustic elements 4 from one another. However, for theexample arrangements shown in FIGS. 2 and 3, the dicing kerfs 14 extendonly partially through the support layer 2, in order to maintain acommon mechanical substrate support for the pillars in the array bymeans of the remaining graphite material.

As indicated in FIGS. 5 and 7, the ultrasound acoustic assembly 100further includes a number of application specific integrated circuit(ASIC) die 32. Each ultrasound acoustic array 40 is coupled to arespective one of the ASIC die 32 to form a respective acoustic-electrictransducer module 10, as indicated in FIG. 5.

For particular arrangements, the ultrasound acoustic arrays 40 may bedisposed directly on the ASIC die 32. That is, the ultrasound acousticarrays 40 may be bonded directly to the ASIC die 32, without aninterposer being disposed in-between the two. For particulararrangements, through silicon vias (TSV) (not shown) may be used toprovide seamless tiling of ASICs and associated transducer stacks. Theuse of TSV's to time ASICs and associated transducer stacks is describedfor the case of capacitive micromachined ultrasonic transducers (cMUTs)sensors in commonly assigned US Patent Application Publication No.20110071397, “Large area modular sensor array assembly and method formaking the same.” which is incorporated by reference herein in itsentirety.

However, for the configuration shown in FIGS. 5 and 7, the ultrasoundacoustic assembly 100 further includes one or more substrates 50disposed between the ultrasound acoustic arrays 40 and the ASIC die 32.As indicated for example in FIG. 7, multiple ultrasound acoustic arrays40 may be coupled to the respective ASIC die 32 via an interposer 50 orflexible substrate 50. For ease of illustration, reference numeral 50 isused to denote both the interposer and the flexible substrate.Non-limiting examples of interposers and flexible substrates includehigh density interconnects.

For the example arrangement shown in FIG. 5, the ASIC die 32 areflip-chip attached to the backside of an interposer 50 by solder bumps38. Further, for the example configuration shown in FIG. 5, pads 39 arecoupled to the interposer 50 for connection to bumps 35 on the acousticarrays 40. Example pads 39 include, without limitation, electrolessnickel immersion gold (ENIG) pads 39. For the configuration shown inFIG. 5, the ultrasound acoustic arrays 40 are attached to the interposer50 by standard reflow solder flip-chip attach. In application, the ASICdie 32 provides the acoustic sensor interface electronics, while theinterposer 50 provides communication from the ASIC die to the transducerarray, as well as from the ASIC die to an external data processingsystem (not shown) through an associated set of I/O ball grid array(BGA) balls 37.

For the example configurations shown in FIGS. 1-5, each acoustic array40 further comprises an under-bump metallization (UBM) 33 connected tothe acoustic elements 4. As noted above and as shown, for example, inFIGS. 2, 3 and 5, the UBM 33 is segmented, with each segment of the UBMattached to an acoustic element 4. In addition, for the exampleconfigurations shown in FIGS. 3-5, each of the acoustic arrays 40further comprises a number of conductive bumps 35 connected to the UBM33 to form a number of raised electrical contacts 34 for the acousticarray 40. Further, for the configurations shown in FIGS. 3 and 5, eachacoustic array 40 further comprises a common ground electrode 6connected to the diced, acoustic stack 18 (FIG. 1). Each of thesefeatures is described above. In addition, the ultrasound acousticassembly 100 may further include a base substrate 110 (FIG. 7), wherethe ASIC die 32 are connected to the base substrate 110. For certainconfigurations, the base substrate 110 may be flexible. For example, thesubstrate 110 may comprise a flexible organic polymer, such aspolyimide, examples of which include materials marketed under the tradename Kapton®, which is commercially available from E. I. du Pont deNemours and Company. Beneficially, a flexible substrate may be shaped tobuild a curved acoustic assembly. One non-limiting example of a curvedacoustic assembly would be a cylindrical acoustic assembly.

As noted above, for the configuration shown in FIG. 7, the ultrasoundacoustic assembly 100 further includes a base substrate 110, where theASIC die 32 are connected to the base substrate 110. For the examplearrangement shown in FIG. 5, the ASIC die 32 are connected to theinterposer 50 by conductive bumps 38. For the configuration shown inFIG. 7, the interposers 50 are connected to the base substrate 100 byI/O ball grid array (BGA) balls 37. Depending on the specificapplication, the base substrate 110 may be rigid or flexible. Forexample, the substrate 110 may comprise a flexible organic polymer, suchas polyimide, examples of which include materials marketed under thetrade names Kapton® and Upilex®. Upilex® is commercially available fromUBE Industries. Ltd. Other exemplary flexible organic polymers includepolyethersulfone (PES) from BASF, polyethyleneterephthalate (PET orpolyester) from E. I. du Pont de Nemours and Company,polyethylenenaphthalate (PEN) from E. I. du Pont de Nemours and Company,and polyetherimide (PEI). Beneficially, a flexible base substrate 110may be shaped to build a curved (for example a cylindrical) acousticassembly.

In addition to the features described above, the ultrasound acousticassembly 100 may further optionally include an electricallynon-conductive material, for example silicone, disposed within thedicing kerfs 14. Beneficially, this material enhances the structuralstability of the diced, acoustic stack, as well as providing acousticisolation and electrical isolation during re-poling and duringoperation. In addition, the ultrasound acoustic assembly 100 may furtheroptionally include an AISC underfill 21 (FIG. 5), for example an epoxyunderfill 21, to help secure the substrate 50 to the ASIC die 32.

Beneficially, the ultrasound acoustic assembly 100 is a relatively lowcost, high yield large area transducer array. The acoustic arrays 40 aredensely integrated with their respective front-end processingelectronics and also have relatively minimal gaps in the uniformdistribution of transducer elements across the entire assembly. Further,the relatively close coupling of the electronics layer to the transducerlayer yields savings in power consumption and improvements in signaldynamic range due to the reduction of parasitic capacitances.

A method of manufacturing an ultrasound acoustic assembly 100 embodimentis described with reference to FIGS. 1-7. As indicated, for example, inFIG. 1, the manufacturing method includes assembling a piezoelectriclayer 42 with at least one acoustic impedance dematching layer 41 andwith a support layer (2) to form an acoustic stack 18. The layers 2, 41,42 are described above. As shown, for example, in FIGS. 2 and 3, themanufacturing method further includes dicing the acoustic stack 18 toform a number of acoustic elements 4, such that a number of dicing kerfs14 are formed between neighboring ones of the acoustic elements 4. Asindicated in FIGS. 2 and 3, for example, the dicing kerfs 14 extendthrough the piezoelectric layer 42 and through the acoustic impedancedematching layer(s) 41, such that the acoustic elements 4 are separatedfrom each other. However, the dicing kerfs 14 extend only partiallythrough the support layer 2, in order to maintain a common mechanicalsubstrate support for the acoustic elements.

As indicated, for example, in FIGS. 5-7, the manufacturing methodfurther includes connecting each of the diced, acoustic stacks 18 to arespective one of a number of application specific integrated circuit(ASIC) die 32, to form a respective acoustic-electric transducer module10. For the processes illustrated in FIGS. 5-7, the connection of thediced, acoustic arrays 18 (FIG. 1) to the respective ASIC die 32comprises disposing the diced, acoustic stacks 18 on one or moresubstrates 50 and coupling the substrates(s) 50 with the ASIC die 32.Example substrates 50 include interposers 50 and flexible substrates 50.For example, after visual inspection and/or testing, the partially-dicedacoustic stacks 18 may be assembled to high density interconnect(s) 50using automated pick and play techniques. Numerically controlledpick-and-place machines (also termed “surface mount technology componentplacement systems”) are known in the art and are typically used to placesurface mount devices (electrical components) on printed circuit boards.Pick-and-place machines typically include pneumatic suction nozzles, andeach nozzle head can be manipulated in three dimensions and rotatedindependently. Typically, the ASIC die 32 will be assembled to theinterposer 50 prior to assembly of the diced acoustic stacks 18 to theinterposer 50. However, for other processes, the ASIC die and the dicedacoustic stacks may be assembled to the interposer(s) 50 in the sameprocess step, and for other processes the diced, acoustic stacks may beassembled to the interposer(s) first and the ASIC die may be assembledto the interposer(s) second. Moreover, for other configurations, thediced, acoustic stacks 18 may be disposed directly on the ASIC die 32.

In addition, the manufacturing method may optionally further includeperforming a reflow operation, such that uniform electrical connectionsare formed between the diced, acoustic stacks 18 (FIG. 1) and therespective ASIC die 32 and interposer 50. For example, the reflowprocess reforms solders bumps 35, such that they provide uniformconnections between the diced, acoustic stacks 18 and the ASIC die32/interposer 50.

The manufacturing method may further optionally include disposing anelectrically non-conductive material into the dicing kerfs 14 afterperforming the reflow operation. Silicone is one non-limiting example ofa suitable electrically non-conductive material. After the material hasbeen deposited, a cure may be performed. Beneficially, this materialenhances the structural stability of the diced, acoustic stack, as wellas providing acoustic isolation and electrical isolation duringre-poling and during operation.

As a result of the high temperatures employed, the PZT may be depoledduring reflow. In which case, the acoustic array may be repoled byapplying a relatively high voltage (e.g. 2 V/micron PZT) for a period oftime (for example, less than about two seconds). Repoling can beaccomplished by grounding all supply terminals on the ASICs and applyingthe relatively high voltage to the common electrode of the transducerelements. A resistor may be added between the repoling voltage and thecommon electrode protects the ASIC from high voltage discharge.Following repoling, the array can be lensed with an acoustic material,such as Silicone.

For the particular process illustrated by FIG. 6, the manufacturingmethod optionally further includes at least partially removing thesupport layer 2 to further acoustically separate the acoustic elements 4from one another. For this process, the support layer 2 is at leastpartially removed prior to the assembly of the electrode 6 to theacoustic array 40. If the support layer is completely removed, the ABSplastic 8 would be deposited on the piezoelectric layer 42. For example,a graphite support layer 2 may be ground away after placement andreflow. Alternatively, the support layer may be released. If the supportlayer is completely removed, the ABS plastic 8 would be deposited on thepiezoelectric layer 42. If the tungsten carbide 41 is partially diced,the manufacturing method may further optionally include disposing waxbetween the remaining tungsten carbide pillars before (at leastpartially) removing the graphite layer 2, and then removing the waxafterwards.

For the particular processes illustrated by FIGS. 5-7, the manufacturingmethod may further optionally include depositing an under-bumpmetallization (UBM) 33 on the acoustic impedance dematching layer 41 andbump plating the acoustic impedance matching layer 41 to form a numberof raised electrical contacts 35. For example, solder bumps 35 may beformed on the UBM 33 to form the electrical contacts 35. As noted above,the process steps may be performed in several different orders. Forexample, the bump plating may be performed after assembling the acousticstack 18. Although, the UBM 33 is typically applied prior to assemblingthe acoustic stack. However, for certain processes, the acousticimpedance dematching layer 41 could be bump plated prior to assemblingthe acoustic stack 18. For certain processes, the bump plating may beperformed before dicing the acoustic stack 18. However, the bump platingcould also be performed after the dicing is completed. For example theUBM could be applied prior to assembling the acoustic stack 18, and theelectrically conductive bumps 35 could be formed after the acousticstack 18 has been diced. Alternatively, and as discussed above, ananisotropic electrically conductive adhesive may take the place of thebump/solder. For other low cost configurations (also not shown),electrically conductive bumps 35 may be deposited directly on theacoustic elements 4 (that is, there is no UBM between the acousticelements and the solder bumps 35).

For the processes illustrated in FIGS. 3, 5 and 6, the manufacturingmethod further optionally includes assembling a common ground electrode6 to each of the respective diced, acoustic stacks 18. Additionaloptional processing steps may be performed. For example, and asdiscussed above, prior to lamination to the piezoelectric layer 42, atungsten carbide acoustic impedance dematching layer 41 may undergosputtering of a layer of titanium (not shown) in order to provide a seedlayer (not shown) for the UBM 33. Next, a layer of nickel may beevaporated on the tungsten carbide acoustic impedance dematching layer41 in order to provide the proper pad for attaching the solder balls 35.This is followed by a very thin flash layer of gold (not shown) toprevent oxidation of the nickel. The tungsten carbide acoustic impedancedematching layer 41 containing the UBM 33 may then be bonded to thepiezoelectric layer 42 and a graphite support layer 2 as thin, solidslabs of material bonded with an adhesive. Suitable nonconductiveadhesives include, without limitation epoxy, polyurethane, and acrylateadhesives and may be bonded using high pressure. Alternately conductiveand anisotropically conducting adhesives may be used to bond thedematching layer to the piezoelectric layer.

Beneficially, the above-described manufacturing method can be used tomanufacture larger area acoustic assemblies at relatively low cost andwith relatively high yield. The manufacturing method facilitates the useof numerically controlled pick-and-place machines to surface mount theacoustic arrays 40 with the graphite support layer 2 (or remainingportion thereof) providing mechanical support for the acoustic pillarsduring the pick-and-place operation.

Another method of manufacturing an ultrasound acoustic assembly (100 isdescribed with reference to FIGS. 8-13. As indicated, for example inFIG. 8, the manufacturing method includes depositing an under-bumpmetallization (UBM) 33 on a number of acoustic impedance dematchingregions (tiles) 41. As indicated in FIG. 9, the manufacturing methodfurther includes disposing a number of electrically conductive bumps 38on a substrate 50. Alternatively and as indicated in FIG. 12, themanufacturing method may include disposing a number of electricallyconductive bumps 38 on application specific integrated circuit (ASIC)die 32. Non-limiting examples of electrically conductive bumps 38include solder bumps, for example, eutectic (lead tin) solder or leadfree solder, as well as gold-stud bumps or Seki-Sui balls. Substrate 50may comprise an interposer 50 or flexible substrate 50. As noted above,reference numeral 50 is used to denote both the interposer and theflexible substrate, for ease of illustration. Non-limiting examples ofinterposers and flexible substrates include high density interconnects.

For the arrangement shown in FIG. 9, the manufacturing method furtherincludes disposing the acoustic impedance dematching regions 41 on thesubstrate 50. For this arrangement, the electrically conductive bumps 35are disposed between the acoustic impedance dematching regions 41 andsubstrate 50. More particularly, for the specific arrangement shown inFIG. 9, partially diced, tungsten carbide tiles 41 are disposed on asubstrate 50. Alternatively and as indicated in FIG. 12, themanufacturing method may further include disposing the acousticimpedance dematching regions 41 on the ASIC die 32, where theelectrically conductive bumps 35 are disposed between the acousticimpedance dematching regions 41 and ASIC die 32. For example, partiallydiced, tungsten carbide tiles 41 may be disposed on a substrate 50. Asused here, the term “disposed” should be understood to encompassarrangements in which a layer is in direct contact with another layer,as well as layers in indirect contact (for example separated by bumps35, 37 or other electrical connection means and/or separated byfillers.)

Although not expressly shown, the manufacturing method illustrated byFIGS. 8-10 further includes performing a reflow operation, such that anumber of uniform electrical connections are formed between the acousticimpedance dematching regions 41 and the respective ASIC die 32 or(intervening) substrate 50. For example, the reflow process reformsconductive bumps 35, such that they provide uniform connections betweenthe acoustic impedance dematching regions 41 and the ASIC die32/substrate 50.

As indicated in FIGS. 8-12, the manufacturing method further includesdisposing at least one piezoelectric region 42 on the acoustic impedancedematching regions 41 to form respective acoustic stacks 18. Inaddition, for the specific arrangements shown in FIGS. 8-10, an optionalacoustic matching layer 2 may be disposed on the piezoelectric region42. As discussed below, the order in which the acoustic layers 41, 42and 2 are assembled and singulated to form acoustic elements 4 varies,depending on the particular process.

For the process illustrated by FIG. 10, the manufacturing method furtherincludes singulating the acoustic stacks 18 to form a number of acousticelements 4. As discussed below, singulation may be performed using anumber of techniques. For the particular process illustrated by FIGS.8-10, the singulation is performed after the acoustic impedancedematching regions 41 and the piezoelectric region(s) 42 have beendisposed within the ultrasound acoustic assembly 100.

For the particular process shown in FIGS. 8 and 9, the manufacturingmethod further includes partially dicing the acoustic impedancedematching regions 41 prior to disposing the acoustic impedancedematching regions 41 on the substrate 50, such that a portion of eachof the respective acoustic impedance dematching regions 41 remains afterthe partial dicing, and where the singulation comprises dicing both thepiezoelectric region(s) 42 and the remaining portion of the acousticimpedance dematching regions 41. For example, the acoustic impedancedematching regions 41 may be partially diced by laser dicing or byetching, for example by performing a dry reactive ion etch (DRIE).Further, the dice may extend about ninety percent, for example, into theacoustic impedance dematching regions 41.

For the process illustrated by FIG. 12, the manufacturing method furtherincludes partially dicing the acoustic impedance dematching regions 41prior to disposing the acoustic impedance dematching regions 41 on theASIC die 32, such that a portion of each of the respective acousticimpedance dematching regions 41 remains after the partial dicing, andwhere the singulation comprises dicing both the piezoelectric region(s)42 and the remaining portion of the acoustic impedance dematchingregions 41. Although the configurations shown in FIGS. 8-12 usepartially diced tungsten carbide tiles 41, for other arrangements, thetungsten carbide tiles 41 are not diced prior to assembly to thesubstrate 50 or to the ASIC die 32.

As indicated in FIG. 11, for example, the manufacturing method mayfurther optionally include disposing epoxy 21 between the acousticimpedance dematching regions 41 and the substrate 50, where the epoxy isdisposed after disposing the partially diced, acoustic impedancedematching regions 41 on the substrate 50. For the arrangement shown inFIG. 12, the manufacturing method would further optionally includedisposing epoxy 21 between the acoustic impedance dematching regions 41and the ASIC die 32, where the epoxy is disposed after disposing thepartially diced, acoustic impedance dematching regions 41 on the ASICdie 32. The epoxy adhesive 21 would be wicked in, thereby helping tosecure the partially diced tungsten carbide tiles 41 to the substrate 50(FIG. 11) or to the ASIC die 32 (FIG. 12), even if the solder bumps 35cannot support the remaining pillars of the partially diced, tungstencarbide tiles 41.

The manufacturing method may further optionally include performing asurface treatment on an upper surface of the acoustic impedancedematching regions 41 after disposing the epoxy. For example, the uppersurface of the entire array of acoustic impedance dematching regions 41may be ground, in order to form a flat surface to facilitate furtherlamination. As indicated in FIG. 13 for example, the manufacturingmethod may further optionally include disposing a gold layer 70 on theupper surface of the acoustic impedance dematching regions 41 afterperforming the surface treatment. For example, a gold layer 70 (forexample, about 1000 Angstroms thick) may be sputtered onto the polishedupper surface of the array of acoustic impedance dematching regions 41to prepare for subsequent lamination.

For particular processes and as shown in FIG. 8, for example, thepiezoelectric region(s) 42 is (are) disposed on the partially diced,acoustic impedance dematching regions 41 and optionally an accousticmatching layer 2, for example graphite, is disposed on the piezoelectricregion(s) 42 prior to disposing the acoustic impedance dematchingregions 41 on the substrate 50 (FIG. 8) or on the ASIC die 32 (FIG. 12).However, as noted above, the order in which the acoustic layers 41, 42and 2 are assembled and singulated to form acoustic elements 4 varies,depending on the particular process. Further, although not expresslyshown, the manufacturing method may further include disposing a numberof optical alignment structures (not shown) on the substrate 50 prior todisposing the acoustic impedance dematching regions 41 on the substrate50. Beneficially, the optical alignment structures may be used to aligna dicing saw or other singulation means to the underlying tungstencarbide grid 41 when dicing the piezoelectric 42 and graphite 2 layers.

Beneficially, the above-described manufacturing methods, acousticarrays, and acoustic assemblies provide relatively low-cost, high yieldmeans for forming larger area acoustic assemblies with integratedfront-end electronics. The resulting integrated acoustic assemblies canbeneficially be used in a variety of applications, such as computedtomography, ultrasound, and digital x-ray, among others.

Although only certain features of the invention have been illustratedand described herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

The invention claimed is:
 1. An ultrasound acoustic assembly comprising:a plurality of ultrasound acoustic arrays, each array comprising anacoustic stack comprising a piezoelectric layer assembled with at leastone acoustic impedance dematching layer and with a support layer,wherein the acoustic stack defines a plurality of dicing kerfs and aplurality of acoustic elements, such that the dicing kerfs are formedbetween neighboring ones of the acoustic elements, and wherein thedicing kerfs extend through the piezoelectric layer and through theacoustic impedance dematching layer(s) but extend only partially throughthe support layer; a plurality of application specific integratedcircuit (ASIC) die, wherein each ultrasound acoustic array is coupled toa respective one of the ASIC die to form a respective acoustic-electrictransducer module wherein each of the acoustic arrays comprises ananisotropic electrically conductive adhesive connected to the acousticelements; and one or more substrates disposed between the ultrasoundacoustic arrays and the ASIC die.
 2. The ultrasound acoustic assembly ofclaim 1, wherein the support layer comprises a material selected fromthe group consisting of graphite, ceramic, silicon, flexible organicpolymer, and combinations thereof.
 3. The ultrasound acoustic assemblyof claim 1, wherein each of the acoustic arrays further comprises: anunder-bump metallization (UBM) connected to the acoustic elements; aplurality of conductive bumps connected to the UBM to form a pluralityof raised electrical contacts for the acoustic array; and a commonground electrode connected to the diced, acoustic stack.
 4. The acousticassembly of claim 1, wherein the acoustic impedance dematching layercomprises a tungsten carbide high impedance dematching layer.
 5. Theacoustic assembly of claim 1, further comprising a base substrateconnected to the ASIC die.
 6. The acoustic assembly of claim 5, whereinthe base substrate comprises a curved, flexible substrate, such that theacoustic assembly comprises a curved acoustic assembly.